Thyristors are nonlinear, solid state devices that are bistable. That is, they have both a high impedance state and a low impedance state. For this reason, thyristors are generally used as solid state switches. Thyristors commonly have four-layer PNPN semiconductor structures, with two intermediate regions called cathode-base and anode-base regions, and two extremity regions adjoining major surfaces of a semiconductor body called cathode-emitter and anode-emitter regions. Thyristors are usually gated or switched from a high impedance blocking state to a low impedance conducting state by applying through a gate electrode a positive electrical control signal to the cathode-base region of the device.
In operation, a positive control signal forward biases the PN junction (cathode junction) between the cathode-emitter and cathode-base regions and causes electron injection into the cathode-base region in the vicinity of the reverse biased, forward blocking center PN junction between the cathode-base and anode-base regions. The injected electrons are swept across the depletion layer at the forward blocking junction, causing an anode-to-cathode electric current and increasing the current gain (.alpha.'s) of the PNP and NPN transistor equivalents of the structure. If the positive control signal is high enough, the sum of the gains (.alpha.s) of the NPN and PNP structure equal unity in some portion or filament, and the device will switch the thyristor from the high impedance, blocking state to the low impedance, conducting state. The thyristor will thereafter remain in the conducting state so long as the current through the thyristor exceeds the holding current of the device.
A major restriction on power thyristors has been the dI/dt capability, i.e. the rate of current increase or "turn-on" as a function of time. The difficulty is that only a small portion of the device is responsive to the control signal and initially switches to the conducting state. The device is dependent upon carrier diffusion to turn-on the remainder of the active regions, which diffusion requires substantial time. Initially, on turn-on, the anode-to-cathode voltage drops instantaneously to about 10% of the blocking state value, and the current is conducted through the portions or filaments of the device in the conducting state, causing a very high current density and localized heating and degrading of the device. To avoid such degradation and possible failure of the thyristor, the external circuit typically requires an inductance to limit the current rise on switching of the thyristor, which causes power losses and generally limits the performance of the circuit.
Amplifying gate thyristors, such as the one shown in FIG. 1, have been developed to provide improved dI/dt capability in power thyristors and to reduce the current carrying requirements of the gate curcuit. An auxiliary or pilot thyristor 2 of annular shape is provided preferably centrally of the main thyristor 1 in the same semiconductor body. Pilot thyristor 2 and main thyristor 1 have their anode-emitter, anode-base and cathode-base regions 3, 4 and 5 in common, and the cathode-emitter regions 6 and 7 of the pilot and main thyristors are spaced adjacent each other along the same major surface of the semiconductor body. A gate electrode 8 is provided adjacent and preferably centrally of the pilot thyristor opposite from the main thyristor, and a floating electrode 9 is provided on the major surface of the semiconductor body astride the PN junction between the cathode-emitter and cathode-base regions of pilot thyristor 2.
Pilot thyristor 2 is turned-on by a control signal applied to gate electrode 8 which flows laterally into pilot thyristor 2 as shown by arrows 10, forward biasing the cathode junction between the cathode-emitter and cathode-base regions 5 and 6, and turning-on the pilot thyristor 2 from the inner edge of the cathode-emitter where the gate current is injected. The resulting anode current as shown by arrows 11 through the pilot thyristor 2 is utilized as a gate current to turn-on main thyristor 1. The anode current from pilot thyristor 2 flows through floating contact 9 and cathode-base region 5 to the cathode shunts 12 along the inner edge of cathode-emitter region 7 of main thyristor 1 as shown by arrows 13. A substantial portion of the main thyristor, particularly if the structure is interdigitated, can thus be initially switched to the conducting state. Such amplifying thyristors can be utilized to rapidly switch high power without substantial power losses. Cathode shunts 12 also provide increase dV/dt capability by conducting anode current without forward biasing the cathode junction and producing a lateral current flow that more rapidly forward biases the cathode junction.
Thyristors are also notorious for their long turn-off times. That is, the time required to establish the high impedance, blocking state in the thyristor on switching from the low impedance, conducting state. In a simple thyristor structure, the blocking state can be restored only by reducing the anode-to-cathode current to below the holding current for such a time period to allow the depletion layer to be reformed at the forward blocking center junction when forward voltage is reapplied. The turn-off time is thus directly related to the diffusion time of the carriers, both electrons and holes, across the base regions and to the carrier lifetimes within the base regions.
Where rapid turn-off capability has been needed, an interdigitated gate electrode structure has been provided with the cathode-emitter region and cathode electrode. A negative control signal is applied to the interdigitated gate electrode(s) to cause a current flow from the cathode electrode and reverse bias the cathode junction between the cathode-base and cathode-emitter regions. The current density in the device can thus be counteracted after rapid reduction of the load current to zero to avoid refiring of the device by rapid reapplication of the load potential, e.g. in high frequency operation of 10 to 20 KHz. Or, the high impedance blocking state can thus be reestablished in the thyristor while a load current is still applied to the device under low frequency or DC load potentials. Thyristors operated in the former mode are commonly called "Gate Assisted Turn-Off Thyristors" or "GATTs"; and thyristors operated in the latter mode are commonly called "Gate Controlled Switches" or "GCSs".
Fast switching power thyristors operated in either the GATT or GCS mode need both gate amplification for fast, low power loss turn-on, and gate assist for fast, low power loss turn-off. However, conventional available amplifying gate thyristors cannot provide both fast turn-on and fast turn-off. In order to perform both fast turn-on and fast turn-off functions in a single device, separate gate electrodes are typically needed for turn-on and turn-off. The two gates are required because amplifying gates have a high lateral resistance (e.g. 10- 25 ohms) in the direction in which current must flow during turn-off. The current for turn-off is of opposite polarity from the gate current for turn-on, and the cathode junction between the cathode-emitter and cathode-base of the pilot thyristor is reverse biased. The current must, therefore, flow laterally through cathode-base region 5 under cathode-emitter region 6 to reach gate electrode 8. The resulting high resistance greatly reduces the effectiveness of the gate turn-off current and in turn degrades the performance of the device. In addition, degradation and failure of the device may occur dure to nonuniform avalanche of the cathode junction between the cathode-emitter and cathode-base regions of the pilot thyristor 2.
Accordingly, a second separate gate is provided for turn-off through which the gate assist current can be shunted to bypass the pilot thyristor. A diode 14 is provided to establish a low resistance path through the second gate bypassing the pilot thyristor in the turn-off mode and, on the other hand, to block gate current from bypassing the pilot thyristor in the turn-on mode. The diode is typically fused to the floating electrode as shown in FIG. 1, which doubles as the separate turn-off gate. The fusion of diode 14 to the floating electrode is deemed necessary to properly align the diode and provide a low resistance path for turn-off. Additional fabrication steps were therefore needed to provide a special thyristor which had both fast turn-on and fast turn-off capability, and special encapsulation and packaging assemblies were needed for such devices.
The present invention overcomes these difficulties and disadvantages and provides a power thyristor with both fast turn-on and turn-off capabilities utilizing a conventional amplifying gate thyristor. A simple, low cost means is provided to adapt existing amplifying gate thyristors and packaging assemblies to provide both gate amplification on turn-on and GATT or GSC modes on turn-off.